1. Field of the Invention
The present invention relates to a bending manufacturing of a long workpiece in which the workpiece is bent while being moved along its length direction.
2. Description of the Background Art
An example of a conventional apparatus for bending manufacturing of a long workpiece can be found in U.S. Pat. No. 4,367,641.
A long workpiece to be manufactured In such an apparatus can be utilized as a seat belt guide for a passive seat belt of a car, for instance. The passive seat belt is a type of seat belt which automatically hold a passenger in a restraint posture when the sitter sits down and closes a door. This is achieved by fixing one end of the seat belt to a console box side of the seat, while attaching another end to a cable side which is slidable in forward and backward direction along the seat belt guide such as that shown in FIG. 1 which is provided in a vicinity of a window sash. The seat belt guide of FIG. 1 has five sections L1, L2, L3, L4, and L5 of different curvatures R1, R2, R3, R4, and R5, respectively, along its length direction, in accordance with a shape of the window sash. Also, as shown in FIG. 2, this seat belt guide has a curvature R6 on its side face, in accordance with a car body shape.
This seat belt guide is manufactured from a workpiece W shown in FIG. 3 which is made of aluminum alloy and which is to be manufactured by extrusion into a form of U-shaped bar having a groove 1 along its length direction along which the cable side of the seat belt is to slide. This workpiece W of FIG. 2 is then bending manufactured in X and Y directions to make the seat belt guide in a form shown in FIG. 1 and FIG. 2. In addition, for some car body shapes, it is necessary to give a twist in a direction .theta..
An apparatus for performing such bending manufacturing of a long workpiece generally comprises a carrier device equipped with carrier rollers for sending out the workpiece along its length direction; a supporting device located in front of the carrier device in a direction in which the workpiece is sent out which is equipped with supporting rollers corresponding to points of support in bending operation, and a bending device located further in front of the supporting device which is equipped with bending rollers to function as points of pressing in bending operation. The bending device is capable of swaying the bending rollers about vertical and horizontal axes, so that a three-dimensional bending manufacturing can be applied to a workpiece.
In such a conventional apparatus for bending manufacturing, a speed of sending out the workpiece is constant throughout an entire process of bending manufacturing. This fact causes no problem for manufacturing of large bending radius sections such as L1 and L3 of the workpiece of FIG. 1 which have relatively large bending radii R1 and R3, respectively. However, in manufacturing of small bending radius sections such as L2 and L4 of the workpiece of FIG. 1 which have relatively small bending radiuss R2 and R4, respectively, sending out of the workpiece at the same speed as manufacturing of small bending radius sections could give rise to a potential danger of a buckling deformation of the workpiece. If the speed of sending out the workpiece is lowered to suit the manufacturing of small bending radius sections, the sending speed is unnecessarily slow for the manufacturing of the large bending radius sections, which causes lowering of a productivity.